JP2011105017A - Inkjet printing head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a printing head in a single-pass inkjet printer having good line merging while avoiding the influence of web weave. <P>SOLUTION: The single-pass printing head, has a plurality of orifice plates each of which serves some but not all of a printing area. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an ink jet print head.

In normal ink jet printing, the print head carries drop-like ink from the orifice to pixel locations in a grid consisting of rows and columns of pixel locations that are closely spaced.
Orifices are often arranged in rows and columns. Since the head rows and columns usually do not cover all the rows or columns in the pixel location grid, the head must scan across the substrate (eg, paper) on which the image is printed.

  To print the entire surface, the print head is scanned across the paper in the head scanning direction, the paper is moved lengthwise to change position, and the head is scanned again at the new position. The line of pixel locations where the orifice prints during scanning is called the print line.

  For a simple mechanism suitable for low resolution printing, during one scan of the print head, the orifices adjacent to the head print along a striped print line representing adjacent rows of the pixel grid. After the stripes of multiple lines are printed, the paper is advanced over the stripes and the next stripe of multiple lines is printed in the next scan.

  High resolution printing provides hundreds of rows and columns per inch (about 2.54 cm) in the pixel grid. A print head usually cannot be composed of a single line of orifices that are sufficiently separated to meet the required print resolution.

  To achieve high resolution scan printing, the orifices in different columns of the printhead are offset or tilted, the printhead scans are superimposed, or the orifices are selectively activated between successive printhead scans. It is possible.

In the system described so far, the head moves in two dimensions relative to the paper (scanning movement along the width of the paper and movement of the paper along its length between scans).
The inkjet head can be as wide as the print area to allow so-called single pass scanning. In single pass scanning, the head is held in a fixed position while the paper is moved along its length in the intended printing direction. All print lines along the length of the paper can be printed in a single pass (paper transport).

  A single pass head may consist of a linear array of multiple orifices. Each of the linear arrays is shorter than the overall width of the print area, and the array is offset to span the entire print width. If the orifice density of each array is less than the required printing resolution, successive arrays are staggered by a small amount along the length of the array to increase the effective orifice density along the width of the paper. By making the print head sufficiently wide to cover the entire width of the substrate, the need to pass back and forth many times is eliminated. It is only necessary to move the substrate along its length in a single pass over the print head. Single-pass printing is faster and simpler than multi-pass printing.

In theory, as long as the substrate is wide, an integrated print head can have a single row of orifices. But that's not really possible for at least two reasons.
The first reason is that for high resolution printing (eg, 600 dpi), the orifice spacing is very small, and at least with current technology, it is mechanically impossible to configure in one line. The second reason is that the production efficiency of the orifice plate drops rapidly as the number of orifices in the plate increases. The reason for this is that there is a non-negligible possibility that a given orifice will have a defect during manufacture or a defect during use. For a print head that has a resolution of 600 inches and has to cover a substrate width of 10 inches (about 25.4 cm), if all the orifices must be in one orifice plate, the manufacturing efficiency is extremely high. Will be lower.
(Overview)
In general, in one aspect, the invention features a single pass inkjet printhead having an array of inkjet outlets sufficient to cover a target width of a print substrate at a predetermined resolution. Each of the plurality of orifice plates has a plurality of orifices. Each of the orifice plates acts on a part rather than the entire printing area. The orifices in the array are printed lines in the array, wherein adjacent parallel lines on the print medium are separated by a distance that is at least an order of magnitude greater than the distance between adjacent orifices perpendicular to the print line direction. Arranged in a pattern to be handled by orifices taking different positions along the direction.

Implementations of the invention may have one or more of the following features. Each orifice plate may be coupled to a print head module that prints a swath along the substrate. The swath is narrower than the target width of the substrate. The number of orifices in each orifice plate is in the range of 250-4000, preferably between 1000 and 2000, and most preferably about 1500. To cover the entire target width, the swath array can be 5 or less (eg, 3).

Other advantages and features will become apparent from the following description and the claims. (Explanation)
The quality of the print produced by a single pass ink jet print head can be improved by selecting the pattern of orifices used to print adjacent print lines. Proper selection of the pattern provides a good trade-off between the effect of web weave and the possibility of print gaps caused by poor line coalescence.

  An object of the present invention is to provide a print head in a single-pass ink jet printer that makes line coalescence good while avoiding the influence of web weave.

The invention according to claim 1 is an inkjet print head for performing printing on a substrate moving in a first direction, in which printing is completed in one pass of the print head for the substrate,
The print head comprises a plurality of linear array modules,
Each linear array module extends along a second direction perpendicular to the first direction, each linear array module acts on a portion of the print area rather than the entire print area, and each orifice on each linear array module corresponds to the first Is arranged to eject ink onto the substrate in a direction along the printing line along the direction of
The orifices of the plurality of linear array modules include a first orifice that acts on a first print line and a second orifice that acts on a second print line adjacent to the first print line. The orifices on adjacent print lines are alternately staggered along the first direction so that they are separated from each other by a first distance along the first direction and a second distance along the second direction. Has been placed,
The ratio of the first distance to the second distance is at least 10;
A first linear array module of the plurality of linear array modules has a first set of multiple orifices in the first direction relative to the first linear array module of the plurality of linear array modules. A second linear array module adjacent along the second set has a plurality of orifices;
Printing handled by the given orifice for a given orifice on the first linear array module and two orifices on either side of the given orifice on the second linear array module. There is at least one print line between a line and a print line handled by one of the two adjacent orifices, and the print line handled by the given orifice and the two adjacent orifices At least one print line is also interposed between the print line handled by the other of
Print head.

Schematic showing web weave. Schematic showing web weave. Schematic showing web weave. Schematic showing union of lines. Schematic showing union of lines. Schematic showing interaction between web weave and line coalescence. A graph of line spread as a function of distance. Schematic of a page moving under a single pass printing head. Schematic diagram of the swath module. Schematic diagram of orifice misalignment arrangement. Graph of misaligned array of orifices. Table of orifice positions. Graph of misaligned array of orifices. The disassembled assembly perspective view which drew the swath module.

  As shown in FIGS. 1 and 2, the paper 10 moved along its length during printing undergoes a so-called “web weave”. Web weave is the tendency of a web (e.g., paper) to move back and forth in a direction 14 perpendicular to the intended printing direction 12 instead of following the trajectory completely along the intended direction 12. Web weaves can reduce the quality of inkjet printing.

  Web weave can be measured in mils / inch. A 0.2 mil / inch weave means that for every inch of movement of the web in the intended direction, the web can move 0.2 mil to one side or the other. As shown in FIGS. 2 and 3, when the inkjet orifices are not arranged in a single line along the paper width, but instead are spaced apart along the intended direction of web motion, the web weave Compared to the distance 15, an adjacency error 17 for the placement of the drops is produced. For example, if the web weave is 0.2 mil / inch and the sensation between adjacent orifices in the web motion direction is 1.5 inches, a 0.3 mil adjacent error in the direction perpendicular to the main direction of motion will result. It can be introduced at a distance between adjacent print lines.

If the only concern is to avoid the effects of web weaves, a good pattern will minimize the spacing along the print line direction between the orifices forming adjacent print lines. In such an arrangement, adjacent lines are printed almost simultaneously and the web weave will have little effect. However, in the case of a head having 12 modules spaced apart along the print line direction (see FIG. 10), the orifices that print adjacent print lines are separated by only one module (eg, , Modules 1, 2,..., 11, 12, 1,. In that case, the final orifice in the pattern is in the twelfth module, 11 modules away from the first orifice in the first module in the second iteration of the pattern.

  As shown in FIG. 2, a pattern that maximizes the distance d between two modules in order to avoid the effects of web weave would work. Modules that print successive pixels in a direction perpendicular to the intended movement of the web are modules 1, 3, 5, 7, 9, 11, 12, 10, 8, 6, 4, and can then return to one. However, as will be explained below, this pattern is not ideal when considering the effects of poor line coalescence. On the other hand, as shown in FIG. 3, the effect of web weave becomes more important when adjacent lines are printed along the intended direction of web movement, for example by modules separated by five.

  As shown in FIG. 4, if all of the pixels in a given region 16 are to be filled by printing several consecutive adjacent lines 18, another cause of poor inkjet print quality. Can occur. As each successive line is printed, a series of drops 20 rapidly coalesce to form a line 22. The line 22 extends across the paper to the sides 24, 26 (in two opposite directions perpendicular to the print line direction). Ideally, the adjacent lines that spread out will eventually reach each other and merge (28) to fill a two-dimensional region (stripe) that extends in both the direction along the line direction and the direction perpendicular to the line direction.

  In the case of non-absorbent web materials, the spread of the line edge is said to be limited by the contact angle (as seen in the cross-sectional view, the contact angle is at the edge where the ink encounters the web surface The angle between the web surface and the ink surface.) As the line widens, the contact angle becomes smaller. When the contact angle reaches the lower limit (for example, 10 degrees), the line spread stops.

  When adjacent lines merge, the contact angle of the line edge decreases. As the contact angle is reduced, the viscosity retarding force is increased and the interfacial tension driving force is decreased, so that the spread speed of the combined stripes to the side is reduced. The reduction in lateral spread can result in white gaps 30 between adjacent lines, each adjacent to the adjacent side opposite the gap.

  The lateral spreading speed of one or more merged print lines varies inversely with the third force of the number of merged lines. According to this rule, when two lines (or stripes) merge to form one stripe, the speed at which the edges of the merged stripes spread laterally is faster than the speed at which the constituent lines or stripes spread. Is 8 times slower. However, when the spread is limited by the contact angle, the spread stops due to the effect of coalescence. Thus, as printing proceeds, various pairs of adjacent lines and / or stripes merge or merge according to the distance between adjacent edges and the spreading speed represented by the number of constituent original lines. There is not. For some pairs of adjacent lines and / or stripes, the spreading speed stops or is small enough to eliminate gaps that have been filled so far. As a result, undesirable permanent unprinted gaps 30 remain unfilled even after the ink has set.

Orifice printing patterns that best reduce the effects of poor line coalescence tend to increase the negative effects of web weaves.
As shown in FIG. 5, ideally, the lines 40, 42, 44, and 46 are simultaneously printed and spread without being merged in order to reduce the influence of line merge failure. A series of parallel gaps 41, 43, 45 remain unfilled. After as much time as possible, the remaining gap will be as narrow as possible, and the remaining lines will intervene taking into account the drop splash diameter that is achieved as a result of the drop splashing when it hits the paper as shown in the figure This is accomplished by bridging the gap with a stream of drops. As a result, a uniform print area without gaps is achieved without requiring additional spread. The droplet splash diameter means the diameter of an ink spot generated in a fraction of a second after the ejected ink droplet hits the substrate until the inertia associated with the ejection of the droplet disappears. During that period, the spread of the drop is governed by the relative effects of inertia (which tends to spread the drop) and viscosity (which tends to prevent spreading). Allowing as much time as possible before placing the intervening drip stream is the adjoining line by orifices spaced as far as possible along the print line direction in the opposite direction to the best direction to reduce the effects of web weave. Will mean an orifice print pattern.

  The effective distance between the orifices that print adjacent lines along the print line direction will compromise web weave and line diffusion factors alternately. Assume that the orifices are often placed in two lines 50, 52 with adjacent orifices as shown in FIG. 6 (this requirement will be relaxed later). I want to find a good distance 54 between the lines. Also assume that the web weave moves the web to the left at a constant speed (considering at least a short distance) W (mil / inch web motion) in the line printing direction. It is also assumed that the line edge 60 spreads away from the center of the printed line at a speed represented by the drop function S (d) (mil / inch). Here, d is the distance from the location where the droplet was ejected onto the paper. FIG. 7 is the distance along the web after ejection of three similar curves 81, 82, 83 versus three different splash diameters of estimated diffusion rates.

  In the example, an important point to consider regarding the printing of drops 62 (FIG. 6) occurs. The drop 62 effectively moves to the right (due to the web weave) in the drawing, and the movement of the edge of the line 60 effectively moves to the right. Initially, when the line is formed from a series of ejected drops, the line edge moves rapidly to the right from the position of the drop 62 with respect to the distance along the web. Accordingly, the overlap between the droplet splash and the diffusion line increases. However, if the line spreading speed decreases while the web weave speed does not decrease for a short distance, the amount of overlap will peak and begin to decrease. The position of the drop 62 where the overlapping portion is maximized is obtained. Maximum overlap occurs when the diffusion rate is equal to the web weave rate.

  In FIG. 7, a horizontal line can be drawn to represent the web weave speed. For a web weave speed of 0.1-0.2 mil / inch represented by lines 68, 69, the intersection with curves 81, 82, 83 occurs independently in the range of 0.8-2.2 inches. .

  As shown in FIG. 8, a print head operable using an orifice printing pattern within the range shown in FIG. 7 has three swath modules 0, 1 and 2 shown schematically. As the three swath modules move the paper in the direction indicated by the arrows, they print three adjacent swaths 108, 110, and 112, respectively, along the length of the paper.

As shown in FIG. 9, each swath module 130 has 12 linear array modules arranged in parallel. Each array module comprises an array of 128 orifices 134 spaced 12/600 inches apart to print at a resolution of 600 pixels / inch across the width of the paper. (The number and shape of the orifices are shown schematically in the figure.)
Assuming that each pixel location across the width of the paper is covered by an orifice that prints one of the required print lines 140 along the length of the paper, as shown in FIG. Modules are arranged in a staggered manner along the length of the array (this discrepancy is not visible in FIG. 9). Thus, as shown in the figure, the first orifice in each module (marked with a large black dot) inherently occupies a position along the width of the paper corresponding to one of the required print lines.

In the bottom array module shown in the figure, the position of the second orifice is indicated by a dot, but the subsequent orifice positions in the array and in other arrays are not shown. Further, FIG. 10 shows a discrepancy pattern for one of the three swath modules, while the other two swath modules have another different discrepancy pattern as shown below.

  FIG. 11 is a graph showing the difference pattern of all three swath modules. The pattern has a jagged profile. Each orifice is upstream or downstream along the printing direction of both adjacent orifices, with one exception, in the transition between swath module 0 and swath module 1. Each swath module graph has dots to indicate which of the first 12 pixels covered by that swath module is handled by the first orifice of each array module. The graph for each swath module simply shows the discrepancy pattern, but not all of the module's orifices. The pattern repeats the pattern shown for each swath module 127 times to the right. Therefore, the 12th pixel in each series is the 0th pixel in the next series. Similarly, the module array numbered 12 in swath module 1 occupies a position of 0 along the Y axis in swath modules 0 and 2 (although not shown in the drawing for clarity).

  FIG. 12 is a table that gives the X and Y positions (inches) of the first orifice of each array module forming the swath module 0 with respect to the position of pixel 1. FIG. 12 demonstrates the discrepancy pattern of the array module. For swath module 0, the pixel location of the first orifice is listed in the column labeled “Pixels”. The module number of the array module to which the first orifice that prints the pixel belongs is indicated in the column labeled “Module Number”. The X position (inches) of the pixel is indicated by the column labeled “X Position”. The Y position of the pixel is indicated by the column marked “Y position”. The swath 2 module is arranged the same as the swath 0 module, and the swath 1 module is arranged the same as the other two modules (rotated 180 degrees) (matches the other two modules).

  The 0.989 inch gap between the final orifice (number 1536) of the swath 0 module and the first orifice (number 1537) of the swath 1 module is that each orifice along the print direction of both adjacent orifices Break the rule of being upstream or downstream. On the other hand, the Y-direction gap between the final orifice (number 3072) of the swath 1 module and the first orifice (number 3073) of the swath 2 module is 4.19 inches, which is good for line merging, but the web weave Not good for.

  Thus, in the example of FIGS. 10-12, the distance along the web direction corresponding to the X-axis of FIG. 7 is relative to all adjacent pairs of print line orifices, excluding pairs spanning the transition between swath modules. 1.2 to 2.0 inches (which is an order of magnitude greater than 1/50 inch, the orifice spacing in any array module, and almost two orders of magnitude greater). Although there are some differences in web direction distances for different orifice pairs, it is desirable to keep the ratio of minimum distance to maximum distance close to 1 in order to derive the maximum benefit from the principles described above. In the case of FIGS. 11 and 12, the ratio is 1.67 (other than two transition pairs).

  The range of distances along the web direction discussed above depends on the speed of web movement along the printing direction, when an ink drop strikes the substrate and when the next adjacent ink droplet strikes the substrate. Means the range of delay time between. If the web speed is 20 inches / second, a distance range of 1.2-2.0 inches moves to a series of durations of 0.06-0.1 seconds.

Each swath module has an orifice plate adjacent to the orifice face of the array module. The orifice plate has a hole stagger pattern that matches the pattern described above. One advantage of the pattern in the table of FIG. 7 is that the orifice plate of swath modules 0, 1 and 2 is rotated 180 degrees compared to the other two. It is the same. Since only one type of orifice plate needs to be designed and manufactured, manufacturing costs are reduced.

  In FIG. 13, the swaths 1 and 2 modules are moved to the left by a position corresponding to 2 pixels compared to the position of FIG. The twelfth pixel (1536) of module 0 and the first pixel (1537) of module 1 are disabled. As a result, the distance along the printing direction increases to 4.589 inches. This distance is bad for web weaves but good for line coalescing.

  FIG. 14 shows the configuration of each swath module 130. The swath module has a manifold / orifice plate assembly 200 and a subframe 202 that together provide a housing for a series of twelve linear array module assemblies 204. Each module assembly includes a piezoelectric body assembly 206, a vibration trap material 207, a conductor assembly 208, a clamp bar 210, mounting washers 213 and 214, and a screw 215. Module assemblies are attached to three groups. The groups are separated by a stiffener 220 attached using screws 222. Two electric heaters 230 and 232 are attached to the subframe 202. The ink inlet mounting portion 240 carries ink from an external tank (not shown) through the subframe 202 to the passage of the manifold assembly 200. From there, the ink is distributed to twelve linear array module assemblies 204, returns to the manifold 200, passes through the subframe 202, exits the mounting 242 and eventually returns to the tank. Screw 244 is used to assemble the manifold to subframe 200. The set screw 246 is used to fix the heater 232. The O-ring 250 provides a seal that prevents ink leakage.

  The number of swath arrays and the number of orifices in each swath array can be determined by the disposal costs associated with the disposal of unusable orifice plates (which is often done when using fewer plates with more orifices) and the number of orifices in the head. It is chosen to provide a good trade-off between swath array assembly and alignment costs (which increases with the number of plates). The ideal trade-off can change with the maturity of the manufacturing process.

  The number of orifices in the orifice plate that provides swaths is in the range of 250-4000, more preferably in the range of 1000-2000, and most preferably about 1500. In one example, the head has three swath arrays, each of which has twelve linear arrays with staggered orifices providing 600 lines / inch across a 7.5 inch print area. At this time, the plate providing each swath array has 1536 orifices.

Other embodiments are within the scope of the claims.
For example, the print head may be a single two-dimensional array of orifices, or any combination of array modules or swath arrays with any number of orifices. For example, the number of swath arrays can be 1, 2, 3, or 5. To ensure good separation along the print line direction between the orifices that print adjacent print lines, the number and spacing of the orifices, the size of the array module, the relative importance of the web weave, line coalescence, and any application Manufacturing costs and other factors will be affected.

  The amount of acceptable web weave is increased for low resolution printing. Ink viscosity and interfacial tension affect the degree of line coalescence, but different inks can be used.

  If the primary concern is web weave or if the primary concern is line coalescence, other orifice patterns can be used.

Claims (1)

An inkjet printhead for printing on a substrate moving in a first direction, where printing is completed in a single pass of the printhead to the substrate,
The print head comprises a plurality of linear array modules,
Each linear array module extends along a second direction perpendicular to the first direction, each linear array module acts on a portion of the print area rather than the entire print area, and each orifice on each linear array module corresponds to the first Is arranged to eject ink onto the substrate in a direction along the printing line along the direction of
The orifices of the plurality of linear array modules include a first orifice that acts on a first print line and a second orifice that acts on a second print line adjacent to the first print line. The orifices on adjacent print lines are alternately staggered along the first direction so that they are separated from each other by a first distance along the first direction and a second distance along the second direction. Has been placed,
The ratio of the first distance to the second distance is at least 10;
A first linear array module of the plurality of linear array modules has a first set of multiple orifices in the first direction relative to the first linear array module of the plurality of linear array modules. A second linear array module adjacent along the second set has a plurality of orifices;
Printing handled by the given orifice for a given orifice on the first linear array module and two orifices on either side of the given orifice on the second linear array module. There is at least one print line between a line and a print line handled by one of the two adjacent orifices, and the print line handled by the given orifice and the two adjacent orifices At least one print line is also interposed between the print line handled by the other of
Print head.

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